1. Field of the Invention
The present invention relates to constant current sources and current regulators, and in particular, to a start-up circuit for use with such constant current sources and regulators.
2. Description of the Related Art
It is well known that self-biased circuits using internal feedback often require some type of start-up circuit to get the circuit started. Such start-up circuits are needed because most self-biased circuits have two stable operating points: one of them being the desired operating point at which the desired amount of current flows, and the other being an undesired zero-current state. Start-up circuits typically supply a small mount of start-up current to the circuit in order to eliminate the undesired zero-current state so that the circuit can get started and stabilize at the desired operating point.
Normally, if the start-up current is chosen to be very small, it will have a negligible effect on circuit operation. However, with the ever increasing precision required in modem circuits, the presence of the start-up current after the circuit has stabilized to the desired operating point can, in many situations, have a detrimental effect on circuit performance. For example, while the LM117 3-Terminal Adjustable Regulator manufactured by National Semiconductor Corporation of Santa Clara, Calif., provides acceptable performance, it does display operational shortcomings in some applications.
Specifically, after the LM117 has settled to its desired operating point, an increase in the input voltage will increase the start-up current, which continues to be supplied to the regulating portion of the circuit. An increase in the input voltage of +5 volts to +15 volts, will not detrimentally affect the performance of the regulator. However, an increase in the input voltage of +1.2 volts or +3 volts to +40 volts, as occurs in modem applications, can increase the start-up current enough to cause a significant change in the currents generated by the current mirror circuitry in the regulating portion of the circuit, thus detrimentally affecting the performance of the regulator. This example illustrates that a start-up circuit for a current regulator is particularly difficult to design because variations in the start-up current supplied to current mirror circuitry can cause fluctuations in the regulated currents.
Thus, there is a need for a current regulator having a start-up circuit which will not degrade the performance of the regulator.